Thermal conductivities of YSZ/Al2O3 composites

The thermal conductivities/diffusivities of YSZ/Al₂O₃ composites have been investigated by a laser flash technique. The thermal conductivity of the composite increases with an increase in the Al₂O₃ volume fraction, and it can be fitted well to the Maxwell theoretical model. The consistency of the thermal conductivities of the composites with the predicted values indicates the absence of obvious interfacial thermal resistances in the composites. The negligible thermal resistance effect from the YSZ and Al₂O₃ grain boundaries is due to the much lower phonon mean free path compared with the grain size in the composite. The low Kapitza resistance of the YSZ/Al₂O₃ interface is discussed in terms of the “clean” and coherent nature of the YSZ/Al₂O₃ interface, together with the small difference between the elastic properties of YSZ and Al₂O₃.     

Thermal conductivities of powder-filled epoxy resins

Effective thermal conductivities of powder-filled epoxy composites were experimentally obtained using a transient method. Fillers used were aluminum and cupric oxide. Comparisons of these data with published correlations indicate that Agari's model can give better fits. Nielsen's model may also give a good prediction of the shape of particles can be determined and values of parameters A and ?m are available. An alternative method of dealing with parameter C₂ in Agari's model is suggested. © 1993 John Wiley & Sons, Inc.     

Model for thermal conductivities in spun yarn carbon fabric composites

A study on the thermal conductivities of spun yarn‐type carbon/phenolic (spun C/P) composites using a thermal‐electrical analogy method is presented. This method is based on the similarity between the partial differential equation that governs the thermal potential and electrical potential distribution. The unit cell of a laminate composite is divided into several conduction elements. By constructing an equivalent thermal resistance network in series, and in parallel based on analogy, we were able to predict the thermal conductivity of the composite. The prediction values obtained from the model are compared with known thermal conductivities on a carbon/epoxy composite with an eight‐harness satin (8HS) texture. It is shown that the model provides a good estimate of the thermal conductivity of the spun yarn fabric‐reinforced composite. With the use of this model, the thermal conductivity of the spun C/P composites with 8HS was validated experimentally. Good agreement was found between the present approach and the experimental results. POLYM. COMPOS., 26:791–798, 2005. © 2005 Society of Plastics Engineers     

Thermal conductivities and Raman spectra of boron-doped carbon materials

Boron-doped carbons were prepared at temperatures from 1000 to 2800°C. The effect of boron doping on the thermal conductivity of carbons has been studied and discussed with the results from Raman scattering. Boron doping above 2200°C depressed the thermal conductivity of carbons and increased the intensity of the 1360-cm⁻¹ Raman band. It appeared that lowering the thermal conductivity is mainly caused by a distortion of the graphite lattice due to boron doping.     

Thermal conductivities from temperature profiles in the polymer electrolyte fuel cell

We report measured temperatures inside the single polymer fuel cell, and thermal conductivities and heat transfer coefficients calculated from these. Temperatures were measured next to the membrane on its two sides, and in the gas channels. Higher temperatures (5 °C or more at 1 A/cm²) were found at the membrane electrode surface than in the gas channels. The thermal conductivity of the membrane (λ^m) was small, as expected from the properties of water and polymer, while the heat transfer coefficient of the electrode surfaces (λ^s) was smaller, 1000±300 W/m² K for a layer thickness of 10 μm. The real coefficient is smaller, since the measured temperatures are systematically smaller than the real ones. The electrode surface heat transfer coefficient is not previously reported. The average value for the catalyst surface plus gas diffusion layer was 0.2 W/m K.     

Effects of stagnant and dispersion conductivities on non-darcian forced convection in square Packed-Sphere channels

This paper presents a numerical and experimental investigation of the effects of stagnant and dispersion conductivities on non-Darcian forced convection in square packed-sphere channels. The theoretical prediction of Nusselt number are found to be in good agreement with the experimental data. A larger near-wall damping of stagnant conductivity is found for the present water-steel medium than for that described by the mixing rule based on the volume fraction. A nonlinear Peclet number dependence, Pe^λ and λ = 0,88, for dispersion conductivity is found to induce better agreement between the theoretical and experimental results especially for the cases of high Peclet number and high D_c/d.     

Thermal dispersion in resin transfer molding

This investigation focuses on the effects of thermal dispersion in resin transfer molding (RTM). A set of volume-average balance equations suitable for modeling mold filling in RTM is described and implemented in a numerical mold filling simulation. The energy equation is based on the assumption of local thermal equilibrium and includes a dispersion term. Thermal dispersion is an enhanced transport of heat due to local fluctuations in the fluid velocity and temperature away from their average values. Nonisothermal mold filling experiments are performed on a center-gated disk mold to investigate and quantify dispersion effects. Good agreement is found between the experimentally measured and numerically predicted temperatures, and a function for the transverse dispersion coefficient in a random glass fiber mat is determined. The results indicate that thermal dispersion is important in RTM processes and must be included in simulations to obtain accurate predictions.     

Automatic quantification of filler dispersion in polymer composites

A facile and objective method is introduced to automatically quantify the filler dispersion in polymer composites through image analysis. This method consists of automatic identification of the fillers in the image and a rigorous measurement of the filler dispersion within the space of functions. Compared with previous methods, this method has the advantages of 1) automatically recognizing the fillers, 2) minimizing the subjectivity induced by the inhomogeneity and noise of the background in the images, 3) a mathematically more rigorous definition of the deviation of the filler dispersion from uniformity, 4) a single performance metric reflecting both the distribution and the size of fillers. Both synthetic and real images from model compounds are used to demonstrate the sensitivity of the proposed method to the dispersion and aggregation of fillers. The computed dispersion index shows good agreement with visual observation of synthetic images and mechanical properties of the model compounds.     

Enhancing the thermal conductivities of SiO2/Epoxy composites by orientation

To improve the thermal conductivity of epoxy resin, tensile way was used to orient the molecular chain of epoxy resin with SiO₂ particles filled. In this article, SiO₂/Epoxy composites which had approximately one‐dimensional lattice structure were prepared. The heat generated by LED chip rapidly passed along the direction of the one‐dimensional orientation in SiO₂/Epoxy composites. The results showed that the thermal conductivity of oriented composites increased with the increase of silica concentration and draw ratio (If S is the cross‐sectional areas of composites at the mold outlet, S₀ is the cross‐sectional areas of composites after molding set, and draw ratio is S/S₀). With the addition of 50 wt% SiO₂ to the epoxy resin, the thermal conductivity of oriented SiO₂/Epoxy composites with the draw ratio of 4 was 0.873 W/m K, which was 2.55 times that of unoriented SiO₂/Epoxy composites. And a thermal conductivity, 5.97 times that of the epoxy resin, was obtained with 80 wt% SiO₂ and the draw ratio of 4. Nevertheless, the relative permittivities of epoxy composites which had 50 wt% SiO₂ with the draw ratio of 4 are stable with increasing frequency. POLYM. COMPOS., 37:818–823, 2016. © 2014 Society of Plastics Engineers     

High-thermal conductivities of epoxy composites via p-phenylenediamine interfacial modification and process intensification

High loadings of fillers are usually needed to achieve high-thermal conductivity (TC) of polymer-based composites, which inevitably sacrifices processability and meanwhile causes high-cost. Therefore, it is of great significance to achieve high-TC composites under low-filler loading. Here, a novel p-phenylenediamine (PPD) modified expanded graphite (EG-PPD)/epoxy (EP) composite with high TC and low-filler content was successfully prepared via pre-dispersion and vacuum assisted mixing strategy. With the improved interfacial compatibility between EG and EP by PPD, the prepared EG-PPD/EP composite exhibited excellent thermal management performance, resulting in the TC of which reached 4.00 W·m⁻¹·K⁻¹ with only 10 wt% (5.59 vol%) of EG-PPD, which is approximately 19 times higher than that of pure EP. Meantime, the interface thermal resistance of EG-PPD/EP composite between EG-PPD and EP is reduced by 33% compared with EG/EP composite. This composite with excellent TC property is expected to be used in thermal management field.     

Effect of dispersion on graphite nanosheet composites

By intercalating and exfoliating natural graphite flakes, expanded graphite was obtained and used as the additive for making composites. The expanded graphite was composed of partly connected graphite nanosheets. Three types of composites were made, representing three levels of dispersing the graphite nanosheets. The first was the impregnation of epoxy resin to the expanded graphite by resin transfer molding. No dispersion was applied, and the expanded graphite can retain its original shape. The second was the use of a high‐power sonication to break apart the expanded graphite. The thickness of the sonicated expanded graphite was reduced to about 100 times. The third method was to use a high‐shear strain rate to separate the graphite nanosheets from the expanded graphite and to disperse them into the resin. The thickness range of the graphite nanosheets was 20–50 nm, about 100 times thinner than the sonicated ones. Compression and impact tests were conducted. The influence of dispersion on the material behavior was studied. Some fracture modes associated with the layered microstructures of the graphite nanosheets were observed. A simple model was used to study the stress transfer and frictional energy consumption of the pullout of the nanosheet. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Raman spectroscopic imaging of graphene dispersion in polymer composites

Polymer composites reinforced by graphene platelets (GPLs) have the advantage of enhanced mechanical properties at low loadings. However the planar geometry of GPLs and their entanglement complicate the dispersion of graphene in the polymer matrix and limit the improvement of the mechanical properties. In this study, confocal Raman spectroscopic imaging was used to map the dispersion of GPLs in an epoxy-based composite and was shown to be a suitable technique to study the dispersion of GPLs in the matrix. The Raman images were then used to explain the observed mechanical behavior of graphene–epoxy composites.     

Thermal diffusivity of nanocarbon composites

This study was performed to measure the thermal diffusivity of different types of nanocarbon composites. Thermoexfoliated graphite (TEG), ultrasonically dispersed TEG, and multiwalled carbon nanotubes were used as fillers in epoxy polymer matrixes. The nanocarbon filler content was 1–10 wt%. The temperature dependence of the thermal conductivity and the heat capacity were extensively characterized in the temperature range between 150 and 425 K. For this purpose, the thermal diffusivity of the composites was investigated by two experimental methods: dynamic λ‐calorimeter and photoacoustic. The comparative analysis of thermal diffusivity of compacted TEG samples with different densities and of nanocarbon‐epoxy with different filler content was carried out. It was found that for the composites with a low distribution of the nanocarbon filler, the thermal diffusivity increases and that the value is determined by the structural and morphological properties of the filler. The orientation function for the TEG‐epoxy composites and the compact TEG samples differs due to the epoxy matrix that reduced anisotropy of the composite. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

Measuring thermal conductivities of anisotropic synthetic graphite–liquid crystal polymer composites

In this study, synthetic graphite particles were added to a liquid crystal polymer and the resulting composites were tested for both the through‐plane thermal conductivity k_thru and the in‐plane thermal conductivity k_in using the transient plane source method. The end use application for these composites is in fuel cell bipolar plate fabrication. The goal of this work was to expand upon a previously developed simple empirical model for the in‐plane thermal conductivity, which is easily measured with the transient plane source method. The results show that the square root of the product of the through‐plane and in‐plane thermal conductivities is an exponential function of the volume percent of filler, ϕ. As the through‐plane thermal conductivity of these composites is accurately predicted with a modified Nielsen model, this empirical relationship can be used to estimate in‐plane thermal conductivities for a range of applications. POLYM. COMPOS., 27:388–394, 2006. © 2006 Society of Plastics Engineers     

Thermal analysis of elastomer composites

Thermal analysis of polychloroprene elastomer composites was carried out. Addition of reinforcing fillers such as precipitated silica (Vulcasil‐S), carbon black (FEF N‐550), and short silk fiber led to significant changes in the degradation pattern, depending on their reinforcement and adhesion ability with the elastomer matrix. Attempts were made to correlate the variations of thermal properties with the surface chemistry and the reinforcement characteristics of these fillers. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 646–651, 2001     

Composition of several types of Safflower seed

The residue remaining after commercial extraction of oil from safflower seed has a greater potential as a source of animal feed or human diet supplement than is presently being realized. Safflower seed hull, kernel, and meal were analyzed to provide more information regarding their nutritive possibilities. Commercial and experimental normal hull varieties and experimental thin hull and striped hull varieties were hand separated into hull and kernel fractions and both fractions analyzed for protein, fat, fiber, ash, and amino acids. Samples of partially decorticated commercial meal and undecorticated meal, hulls, and defatted kernel from striped hull seeds were analyzed for protein, fat, fiber, ash, lignin, pentosans, anhydrouronic acid, total and reducing sugars, and amino acids. Cellulose was calculated by difference. A new factor for converting nitrogen to protein for summative analyses of safflower seed was calculated. These analyses indicate that about 15% of the nonfiber, nonash, nonprotein part of the defatted safflower kernel is of unknown composition. W. Utiliz. Res. Dev. Div., ARS, USDA.     

Thermal conductivities of electrospun polyimide‐mesophase pitch nanofibers and mats

In this article, we report the preparation and thermal properties of polyimide–mesophase pitch (MP) composite nanofibers and associated nanofiber nonwoven mats produced using an electrospinning process. The addition of MP increased the thermal conductivities of both the individual composite nanofibers and the in‐plane conductivities of the nanofiber mats. The out‐of‐plane conductivity of the mats remained relatively low due to low through thickness connectivity between the nanofibers. These nanofiber mats are flexible and very thin and are good candidates for thermal management films for future flexible electronic devices. POLYM. ENG. SCI., 54:977–983, 2014. © 2013 Society of Plastics Engineers     

Longitudinal dispersion in oblong aerated systems

A scale model study to the flow and mixing phenomena has been carried out in oblong aeration basins, where a transversal circulating flow of the liquid is introduced by dispersing air along one side of the basin. A semi-empirical correlation of dimensionless numbers has been developed, which is considered to give a reasonable prediction of the rate of longitudinal mixing. Experiments carried out in commercial basins have shown to be in good agreement with the presented model.